With the opening today of its 12-mile-long Blue Line from National Airport to RFK Stadium, Washington's Metro subway grows from a downtown demonstration line into the spine of a regional transportation system that could rival the Capital Beltway in its effect on Washington.

On the new line, commuters will be able to cross the Potomac River without bridge bottlenecks, bureaucrats can shuttle among major federal employment centers without traffic jams, travelers can go to National Airport without having to take a cab, and Capitol Hill aides will be able to lunch at restaurants around Dupont Circle.

More important, the success or failure in the pass few months of the Washington Metro may well [WORD ILLEGIBLE] mine whether subway construction has a [WORD ILLEGIBLE] not only in Washington but in the rest of the nation as well, transit experts agree.

"I would have to say that thisis a critical test for us," said former Metro general manager Warren Quenstedt. "If this one works - particularly after we add the line to Silver Spring in November - then I think Metro's long-term funding problems will be over."

Quenstedt, now a Metro consultant, did not say what would happen if the Blue Line does not attract large numbers of riders and thus generate political support in the nation's capital to complete all - or most - of the planned 100-mile Metro.

The fact is that Metro is in financial trouble. It has construction money assured for only 60 miles [WORD ILLEGIBLE] systems the costs of operating public transit [WORD ILLEGIBLE] are falling increasingly and unpopularly on the auto-driving homeowner's property tax, and perhaps most important, the White House is all but openly hostile to subway construction.

"I suspect that many of the rapid transit systems are grossly overdesigned," President Carter wrote Transportation Secretary Brock Adams in March. "We should insist on (a) off-street parking, (b) one-way streets, (c) special bus lines, (d) surface rail/bus as preferable alternatives to subways. In some urban areas, no construction at all would be needed if a, b and c are required."

Somebody else's subway is also poisoning the well. The other big new system in the country, San Francisco's Bay Area Rapid Transit (BART), is accumulating large losses and has not relieved the flood of automobiles in that spectacular city.

"We're already getting better cost recovery than BART," Metro general manager Theodore Lutz said in an interview. "They're only getting a third of their operating revenue; our projections show 44 per cent right now."

In fact, Metro Transit authority members were told yesterday that operations on the existing Red Line between Dupont Circle and Rhode Island Avenue NE are costing $5.7 million less than anticipated because of favorable ridership and lower-than-expected costs.

The bad news, however, is that Metro will lose a total of $76 million in bus and subway operations in the fiscal year beginning today. Subway revenues will continue to increase as more of the system opens. However, even those who favor Metro argue heatedly over whether revenue ever will equal operating costs.

And the expense of the putting the subway in the ground is enormous. The 17 miles and 24 stations that are now operating cost $1.2 billion.

The estimated cost of the entire projected 100-mile system has escalated from $2.5 billion to more than $5 billion since ground was broken in December, 1969. "Everything else has doubled, too. We should be measured against inflation generally," Quenstedt argued.

Officials at the Urban Mass Transportation Administration (UMTA), one of Secretary Adams' agencies, do not entirely disagree.

"It's time people stop using BART as the example," said UMTA's acting administrator, Charles Bingman. "BART is an attempt to combine commuter rail with downtown subway." Bingman didn't say so, but the implication is that the combination hadn't worked well.

"Metro has a superb downtown distribution system, much better than BART's," said George Wickstrom, a transit expert at the Metropolitan Washington Council of Governments and the staff director of a regional study on how to build the rest of the Metro system.

That "superb system" has already resulted in a bonus Metro did not expect: a third rush hour - at noon - on the existing Red Line. Whether that third rush hour will continue as Metro stretches into the suburbs remains an open question.

"Do you ride the train three miles to eat lunch at another McDonald's?" asked Fairfax County Supervisor Marie B. Travesky.

Those questions are being addressed during some very tough regional meetings currently under way at COG. The area politicians and technicians who attend those meetings are working on the "alternatives analysis" - the restudy of the Metro system beyond 60 miles.

That restudy was ordered by UMTA and the parent Department of Transportation to see whether cheaper transportation would not work as well. Similar studies have been ordered in 20 other communities in the United States, and they are all competing for federal mass transit dollars.

Atlanta, Miami and Baltimore have subway construction or plans well under way. Buffalo is building a "light rail" or super trolley. Honolulu, Detroit, San Juan and Los Angeles are all in various stages of waiting in line.

"We're just terribly concerned about all these systems, and that they be properly managed," Secretary Adams said recently. "The problem is how do we fund them all. To us it's a management problem."

Adams insists in all his statements that the Carter administration is a friend of mass transit, and that mass transit will be a necessary alternative to the automobile as energy and environmental considerations continue to grow.

"We have to get those alternatives in place," he has said.

Adams holds in his hands the federal money that is essential for urban mass transit systems. Under the formula that applies now, 80 per cent of construction costs are federal, 20 per cent are local.

Whether Metro furnishes support for those who favor rail construction or is a hindrance will depend largely on local politicians.

The Metro alternatives analysis is scheduled for completion in October. The regional task force will recommend whether lines such as the one from Arlington to Vienna in the Interstate 66 corridor should be built.

The study will project future Metro ridership and answer such questions as how many potential riders are lost because there is not enough parking at some stations?

The central overriding question that the alternatives analysis must answer is: How do we pay for the operation of this system?

Metro's finances have been hand-to-mouth, day-to-day. Just yesterday the Metro board went to the 11th hour before agreeing to a contract with Fairfax County that was necessary to start subway operations today.

A long-term financial plan, falling from the definition of what the Metro system will really be, is essential. Many, particularly Metro board vice chairman Joseph Wholey, feel that a regional tax earmarked for the operating subsidy also will be essential.

The best selling point for Metro is the train itself. "It's really beautiful," UMTA's Bingman said after his first trip on the Blue Line Wednesday.